Osmosis - Real-life applications

Photo by: Yali Shi

Cell Behavior and Salt Water

Cells in the human body and in the bodies of all living things behave
like microscopic bags of solution housed in a semipermeable membrane.
The health and indeed the very survival of a person, animal, or plant
depends on the ability of

I
NTHIS
1966
PHOTO
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A GIRL IS UNDERGOING KIDNEY DIALYSIS
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A CRUCIAL MODERN MEDICAL APPLICATION THAT RELIES ON OSMOSIS
.

(Bettmann/Corbis

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Reproduced by permission.)

the cells to maintain their concentration of solutes.

Two illustrations involving salt water demonstrate how osmosis can
produce disastrous effects in living things. If you put a carrot in
salty water, the salt water will "draw" the water from
inside the carrot—which, like the human body and most other
forms of life, is mostly made up of water. Within a few hours, the
carrot will be limp, its cells shriveled.

Worse still is the process that occurs when a person drinks salt
water. The body can handle a little bit, but if you were to consume
nothing but salt water for a period of a few days, as in the case of
being stranded on the proverbial desert island, the osmotic pressure
would begin drawing water from other parts of your body. Since a human
body ranges from 60% water (in an adult male) to 85% in a baby, there
would be a great deal of water available—but just as clearly,
water is the essential ingredient in the human body. If you continued
to ingest salt water, you would eventually experience dehydration and
die.

How, then, do fish and other forms of marine life survive in a
salt-water environment? In most cases, a creature whose natural
habitat is the ocean has a much higher solute concentration in its
cells than does a land animal. Hence, for them, salt water is an
isotonic solution, or one that has the same concentration of
solute—and hence the same osmotic pressure—as in their
own cells.

Osmosis in Plants

Plants depend on osmosis to move water from their roots to their
leaves. The further toward the edge or the top of the plant, the
greater the solute concentration, which creates a difference in
osmotic pressure. This is known as osmotic potential, which draws
water upward. In addition, osmosis protects leaves against losing
water through evaporation.

Crucial to the operation of osmosis in plants are "guard
cells," specialized cells dispersed along the surface of the
leaves. Each pair of guard cells surrounds a stoma, or pore,
controlling its ability to open and thus release moisture.

In some situations, external stimuli such as sunlight may cause the
guard cells to draw in potassium from other cells. This leads to an
increase in osmotic potential: the guard cell becomes like a person
who has eaten a dry biscuit, and is now desperate for a drink of water
to wash it down. As a result of its increased osmotic potential, the
guard cell eventually takes on water through osmosis. The guard cells
then swell with water, opening the stomata and increasing the rate of
gas exchange through them. The outcome of this action is an increase
in the rate of photosynthesis and plant growth.

When there is a water shortage, however, other cells transmit signals
to the guard cells that cause them to release their potassium. This
decreases their osmotic potential, and water passes out of the guard
cells to the thirsty cells around them. At the same time, the
resultant shrinkage in the guard cells closes the stomata, decreasing
the rate at which water transpires through them and preventing the
plant from wilting.

Osmosis and Medicine

Osmosis has several implications where medical care is concerned,
particularly in the case of the storage of vitally important red blood
cells. These are normally kept in a plasma solution which is isotonic
to the cells when it contains specific proportions of salts and
proteins. However, if red blood cells are placed in a hypotonic
solution, or one with a lower solute concentration than in the cells
themselves, this can be highly detrimental.

Hence water, a life-giving and life-preserving substance in most
cases, is a killer in this context. If red blood cells were stored in
pure water, osmosis would draw the water into the cells, causing them
to swell and eventually burst. Similarly, if the cells were placed in
a solution with a higher solute concentration, or hypertonic solution,
osmosis would draw water out of the cells until they shriveled.

In fact, the plasma solution used by most hospitals for storing red
blood cells is slightly hypertonic relative to the cells, to prevent
them from drawing in water and bursting. Physicians use a similar
solution when injecting a drug intravenously into a patient. The
active ingredient of the drug has to be suspended in some kind of
medium, but water would be detrimental for the reasons discussed
above, so instead the doctor uses a saline solution that is slightly
hypertonic to the patient's red blood cells.

One vital process closely linked to osmosis is dialysis, which is
critical to the survival of many victims of kidney diseases. Dialysis
is the process by which an artificial kidney machine removes waste
products from a patients' blood—performing the role of a
healthy, normally functioning kidney. The openings in the dialyzing
membrane are such that not only water, but salts and other waste
dissolved in the blood, pass through to a surrounding tank of
distilled water. The red blood cells, on the other hand, are too large
to enter the dialyzing membrane, so they return to the
patient's body.

Preserving Fruits and Meats

Osmosis is also used for preserving fruits and meats, though the
process is quite different for the two. In the case of fruit, osmosis
is used to dehydrate it, whereas in the preservation of meat, osmosis
draws salt into it, thus preventing the intrusion of bacteria.

Most fruits are about 75% water, and this makes them highly
susceptible to spoilage. To preserve fruit, it must be dehydrated,
which—as in the case of the salt in the meat—presents
bacteria with a less-than-hospitable environment. Over the years,
people have tried a variety of methods for drying fruit, but most of
these have a tendency to shrink and harden the fruit. The reason for
this is that most drying methods, such as heat from the Sun, are
relatively quick and drastic; osmosis, on the other hand, is slower,
more moderate—and closer to the behavior of nature.

Osmotic dehydration techniques, in fact, result in fruit that can be
stored longer than fruit dehydrated by other methods. This in turn
makes it possible to provide consumers with a wider variety of fruit
throughout the year. Also, the fruit itself tends to maintain more of
its flavor and nutritional qualities while keeping out microorganisms.

Because osmosis alone can only remove about 50% of the water in most
ripe fruits, however, the dehydration process involves a secondary
method as well. First the fruit is blanched, or placed briefly in
scalding water to stop enzymatic action. Next it is subjected to
osmotic dehydration by dipping it in, or spreading it with, a
specially made variety of syrup whose sugar draws out the water in the
fruit. After this, air drying or vacuum drying completes the process.
The resulting product is ready to eat; can be preserved on a shelf
under most climatic conditions; and may even be powdered for making
confectionery items.

Whereas osmotic dehydration of fruit is currently used in many parts
of the world, the salt-curing of meat in brine is largely a thing of
the past, due to the introduction of refrigeration. Many poorer
families, even in the industrialized world, however, remained without
electricity long after it spread throughout most of Europe and North
America. John Steinbeck's
Grapes of Wrath
(1939) offers a memorable scene in which a contemporary family, the
Joads, kill and cure a pig before leaving Oklahoma for California. And
a Web site for Walton Feed, an Idaho company specializing in
dehydrated foods, offers reminiscences by Canadians whose families
were still salt-curing meats in the middle of the twentieth century.
Verla Cress of southern Alberta, for instance, offered a recipe from
which the following details are drawn.

First a barrel is filled with a solution containing 2 gal (7.57 l) of
hot water and 8 oz (.2268 kg) of salt, or 32 parts hot water to one
part salt, as well as a small quantity of vinegar. The pig or cow,
which would have just been slaughtered, should then be cut up into
what Cress called "ham-sized pieces (about 10-15 lb [5-7 kg])
each." The pieces are then soaked in the brine barrel for six
days, after which the meat is removed, dried, "and put…
in flour or gunny sacks to keep the flies away. Then hang it up in a
cool dry place to dry. It will keep like this for perhaps six weeks if
stored in a cool place during the Summer. Of course, it will keep much
longer in the Winter. If it goes bad, you'll know it!"

Cress offered another method, one still used on ham today. Instead of
salt, sugar is used in a mixture of 32 oz (.94 l) to 3 gal (11.36 l)
of water. After being removed, the meat is smoked—that is,
exposed to smoke from a typically aromatic wood such as hickory, in an
enclosed barn—for three days. Smoking the meat tends to make it
last much longer: four months in the summer, according to Cress.

The Walton Feeds Web page included another brine-curing recipe, this
one used by the women of the Stirling, Alberta, Church of the
Latter-Day Saints in 1973. Also included were reminiscences by Glenn
Adamson (born 1915): "…When we butchered a pig, Dad
filled a wooden 45-gal (170.34 l) barrel with salt brine. We cut up
the pig into maybe eight pieces and put it in the brine barrel. The
pork soaked in the barrel for several days, then the meat was taken
out, and the water was thrown away…. In the hot summer days
after they [the pieces of meat] had dried, they were put in the root
cellar to keep them cool. The meat was good for eating two or three
months this way."

For thousands of years, people used salt to cure and preserve meat:
for instance, the sailing ships that first came to the New World
carried on board barrels full of cured meat, which fed sailors on the
voyage over. Meat was not the only type of food preserved through the
use of salt or brine, which is hypertonic—and thus
lethal—to bacteria cells. Among other items packed in brine
were fish, olives, and vegetables.

Even today, some types of canned fish come to the consumer still
packed in brine, as do olives. Another method that survives is the use
of sugar—which can be just as effective as salt for keeping out
bacteria—to preserve fruit in jam.

Reverse Osmosis

Given the many ways osmosis is used for preserving food, not to
mention its many interactions with water, it should not be surprising
to discover that osmosis can also be used for desalination, or turning
salt water into drinking water. Actually, it is not osmosis, strictly
speaking, but rather reverse osmosis that turns salt water from the
ocean—97% of Earth's water supply—into water that
can be used for bathing, agriculture, and in some cases even drinking.

When you mix a teaspoon of sugar into a cup of coffee, as mentioned in
an earlier illustration, this is a non-reversible process. Short of
some highly complicated undertaking—for instance, using
ultrasonic sound waves—it would be impossible to separate
solute and solvent.

Osmosis, on the other hand, can be reversed. This is done by using a
controlled external pressure of approximately 60 atmospheres, an
atmosphere being equal to the air pressure at sea level—14.7
pounds-per-square-inch (1.013 × 10
5
Pa.) In reverse osmosis, this pressure is applied to the area of
higher solute concentration—in this case, the seawater. As a
result, the pressure in the seawater pushes water molecules into a
reservoir of pure water.

If performed by someone with a few rudimentary tools and a knowledge
of how to provide just the right amount of pressure, it is possible
that reverse osmosis could save the life of a shipwreck victim
stranded in a location without a fresh water supply. On the other
hand, a person in such a situation may be able to absorb sufficient
water from fruits and plant life, as Tom Hanks's character did
in the 2001 film
Cast Away.

Companies such as Reverse Osmosis Systems in Atlanta, Georgia, offer a
small unit for home or business use, which actually performs the
reverse-osmosis process on a small scale. The unit makes use of a
process called crossflow, which continually cleans the semipermeable
membrane of impurities that have been removed from the water. A small
pump provides the pressure necessary to push the water through the
membrane. In addition to an under-the-sink model, a reverse osmosis
water cooler is also available.

Not only is reverse osmosis used for making water safe, it is also
applied to metals in a variety of capacities, not least of which is
its use in treating wastewater from electroplating. But there are
other metallurgical methods of reverse osmosis that have little to do
with water treatment: metal finishing, as well as recycling of metals
and chemicals. These processes are highly complicated, but they
involve the same principle of removing impurities that governs reverse
osmosis.

User Contributions:

This website is wonderful. I have been completing an experimental report on the topic of hypertonic and hypotonic solutions and your website provided information that i was unable to find anywhere else on the web. Thank you very much.

I am trying to discover the method by which vegetables can be dehydrated without loss of volume (shrinkage) or color change. I recently came across a farmer's stand selling them, but the recipe was held "secret". They were crunchy and delicious and brightly colored. Included were green beans, potatoes, carrots, sweet potatoes, corn and the like. Lightly salted, they are much like potato chips in appeal. I live in a different state, and grow most of my own food, and feel it is important for people to know how to preserve their own food. This should not be an industry secret. I do think that it involves osmotic dehydration, using a saline solution. The final product in my "find" also contained canola oil, apparently as a last step. I am guessing this might be sprayed on at the end to improve color. Do you have any ideas on this process?

Last year I did some experiments on salt curing vegetables. I do believe this is the first step, with air-drying as a follow up. But I am looking for more specific information. One thing for sure, there is lots of research out there, but it is in scientific journals, and not in cookbooks. That's how I found out that aubergine is eggplant!